1. Field
The present disclosure relates generally to wireless communication devices and, more specifically, to a method of estimating the I/Q gain mismatch and phase mismatch simultaneously in the transmit and receive paths of a radio transceiver of an OFDM FDD system.
2. Background
Transceivers for wireless communication systems that use quadrature amplitude modulation typically exhibit I/Q gain and phase mismatches. A data signal that is to be transmitted by a communication system employing quadrature modulation is first converted into an in-phase (I-phase) transmit component and a quadrature-phase (Q-phase) transmit component. Then in the analog radio transmitter, the I and Q components of the data signal are up-converted to a radio frequency (RF) band. An up-converted. I-phase RF signal is generated when the I-phase transmit component is mixed with an I-phase component of a local oscillator signal. Similarly, an up-converted. Q-phase RF signal is generated when the Q-phase transmit component is mixed with a Q-phase component of the local oscillator signal. The I-phase and Q-phase RF signals are then added to form a composite RF signal that is transmitted over an air interface. Phase and gain imbalances between I and Q branches of the transmitter are introduced when the I-phase and the Q-phase components of a local oscillator signal are not separated by exactly 90 degrees and when the amplitude of the up-converted. I-phase RF signal docs not exactly equal the amplitude of the up-converted. Q-phase RF signal. In a similar manner, phase and gain imbalances between I and Q branches of the analog radio receiver are introduced when the received RF composite signal is down-converted by mixing with the I-phase and Q-phase components of a local oscillator signal. The phase and gain imbalances between I and Q branches of the receiver occur when the I-phase and Q-phase components of a local oscillator signal in the receiver are not separated by exactly 90 degrees and when the amplitude of the down-converted I-phase signal does not exactly equal the amplitude of the down-converted Q-phase signal. The four impairments caused by the phase and gain imbalances in the transmitter and receiver cause interference between positive and negative side bands of the signal spectrum.
Some wireless communications systems that are based on orthogonal frequency-division multiplexing (OFDM) and time division duplexing (TDD) currently include functionality to estimate the I/Q gain and phase impairments in the transmitter and in the receiver. The existing methods of estimating I/Q mismatches, however, have various disadvantages. First, the existing methods are performed in two steps. The transmit path must be calibrated before the receive path can be calibrated. The transmit I/Q mismatches are estimated; then the transmit path is corrected; and finally the calibrated transmit path is used to provide a reference signal for estimating the I/Q mismatches of the receive path. Second, the existing methods require additional hardware that is not otherwise used in the regular transmission of data signals in current OFDM modern transceivers, such as those used for the WLAN and WiMax standards. Separate dedicated hardware is needed to estimate the mismatch in the transmit path. Typically this hardware is an RF envelope detector. After the I/Q mismatches in the transmit path are estimated using the dedicated hardware, the local oscillator and mixers in the receiver are used to estimate the I/Q mismatches in the receiver.
A method is sought for estimating the I/Q gain and phase impairments in both the transmitter and the receiver of an OFDM transceiver at one time. In addition, an apparatus is sought that can estimate the I/Q gain and phase impairments in both the transmitter and receiver of an OFDM transceiver without requiring significant hardware not otherwise used in the regular transmission of data signals.